1. Preheat oven to 375 degrees. Brush a 13-by-9-inch baking dish with butter; set aside. In a medium saucepan, combine 3 cups sugar, lemon juice, and 1 1/2 cups water; bring to a boil. Reduce heat; simmer until sugar dissolves and mixture is syrupy, about 10 minutes. Set syrup aside.

2. In a food processor, pulse walnuts with cinnamon and remaining 1/2 cup sugar until finely ground. Set walnut filling aside.

3. Place stack of thawed phyllo sheets on a work surface. Using a sharp knife, trim stack into a 13-by-9-inch rectangle; discard trimmings. Place 1 sheet of trimmed phyllo in prepared baking dish (keep remaining sheets covered with a damp cloth or paper towel). Brush gently with butter; repeat with two more sheets of phyllo, laying each on top of the other. (You now have a 3-sheet stack of buttered phyllo.)

4. Sprinkle phyllo stack in dish with 1/3 cup walnut filling. Repeat with 7 more buttered phyllo stacks, sprinkling each with walnut filling. Top with one more stack; brush generously with butter.

5. Using a sharp knife, cut unbaked baklava into 24 squares. Bake until puffed and golden, 30 to 35 minutes. Transfer pan to a wire rack; pour syrup over baklava. Let stand at room temperature until syrup is absorbed, at least 3 hours.